The present invention relates to wireless communications, and more particularly, to a system for establishing two-way packet data communications, preferably by satellite, between a control station, the earth station in a satellite-based system, and a plurality of widely dispersed remote terminals. The system may be used to provide a communications conduit between a remote information source and/or sink and a fixed controlling and/or monitoring station.
Satellites at a geostationary orbit, located approximately 22,000 miles above the equator, may be used to provide one-way or two-way communications with a widely dispersed remote terminal population, some of which may be affixed to mobile assets. There are numerous existing satellite systems, providing a variety of voice and data services. Communication transmissions from the earth station of the present invention to the plurality of terminals is hereinafter referred to as the forward link, while communication transmissions from the terminal to the earth station is hereinafter referred to as the return link.
For many existing communication systems, such as cellular systems, individual terminals transmit and/or receive over their communications channel only sporadically, being active for typically less than 1% of the time. During periods of communication inactivity, many of these terminals still continue to operate at power consumption levels not markedly different from periods of active communications due to the need to exchange housekeeping data with a base station. When the capacity of the terminal's source of power is large relative to the terminal nominal power consumption, such as may be the case when the terminal is powered through an AC connection, inefficient use of power may be tolerable. For those cases where the source of terminal power is of limited capacity however, such as where batteries provide the power source, heightened efficiencies in power consumption are desirable to reduce the frequency of terminal communication outage periods and/or inconveniences associated with having to replace or recharge its batteries.
There are numerous examples of communication systems where terminals operating within a system network must be readily available for communications with its network controller. For such a system, a terminal may be required to continually monitor its receive channel and hence endure significant power consumption during periods of communications inactivity.
A method known by those skilled in the art for reducing the average power consumption at a remote terminal is to cycle the terminal between active and inactive states. The active state typically includes a receive mode in which the terminal monitors the forward link communications channel, a transmit mode in which the terminal transmits a return link signal via the return communications channel, and a terminal processing mode in which the terminal's processing engine, such as a microcontroller or microprocessor is active. The inactive state is often referred to as the sleep mode in which power is removed from one or more terminal components, and typically has a power consumption that is significantly less than for any of the active modes. The savings in power consumption, however, occur at the expense of terminal communications availability. For example, transmissions from an earth station will not be received and processed by the remote terminal if it is inactive during the transmission period. For such a case, the terminal is said to have a low availability. One solution to a terminal's reduced availability is to have the earth station transmit the same message numerous times to the terminal. In this case, the low power consumption achieved by the terminal using sleep mode occurs at the expense of bandwidth efficiency.
The primary purpose of a communications channel is to enable two or more entities to communicate, such as between a remote terminal and an end-customer. The forward and return link signals may include aids such as pilot signals to help terminals become synchronized with the earth station in both time and frequency, and a modest amount of communications overhead between a terminal and its network controller to facilitate network access by the terminal. The communications overhead between the network controller and terminals may include identification of a terminal or a periodic update of network parameters.
It is acknowledged by those in the art that it is desirable to minimize the communications overhead, and to thereby maximize the efficiency of the utilization of the communications channel. Mandatory periodic communications between each terminal and a base station, such as occurs in cellular systems, results in inefficient use of the communications channel, particularly for those applications where the nominal sleep period for communications between a terminal and end-user is significantly greater than the period of the mandatory communications.
U.S. Pat. No. 5,392,287 discloses a method of reducing the receiver power consumption for a communications system having a number of remote terminals. The system enables the receivers of each of the remote terminals to periodically enter an active state during which time they can receive messages addressed to it on a paging channel. The periodicity of the active state of the prior art is limited to be in the range from 2 to 128 seconds. There are a number of applications where communications between a remote terminal and the earth station is required on a much less frequent basis, such as on a monthly or daily basis. For such applications, the battery lifetime of the terminals using the concept of U.S. Pat. No. 5,392,287 would be unduly limited.
Furthermore, the transmitter of the earth station of the prior art, referred to therein as the base station, is required to transmit one or more messages to each terminal during each occurrence of its active state. This results and considerable power drain since the terminal must receive a block of data. Furthermore, while it may be desirable to have a terminal monitor the communications channel periodically to determine if there any messages or packets, it is not desirable in terms of the bandwidth efficiency to have the earth station transmit a message at each occurrence of a terminal's active state dedicated to that terminal. The communications overhead associated with the requirement to transmit a dedicated message to each terminal during its active state severely limits the capacity of the system.
Satellite communications is often used as the communications conduit between remote sensors, which may sense infrequently, and a monitoring station. For a number of those applications, it may be desirable to be able to remotely change the frequency with which the sensors are sensed without requiring physical access to the terminal or causing the remote terminal to become unavailable during periods of reconfiguration.
An object of the invention is to alleviate the aforementioned disadvantages of the prior art.